Hey Mattraws97,
Thanks for the reply and additional information, we appreciate it!
You are correct, when using Sync Library, changes to your music library on one device will also appear on another, as long as you're using the same Apple ID and Sync Library is enabled. Turn on Sync Library with Apple Music
When Sync Library is enabled, using iTunes or Finder (with macOS Catalina) is disabled. So I'm not sure how you were able to sync your iPhone with music from your library with your Mac. If you have a third-party app for syncing, that could possibly by why, and therefore interfere with Sync Library.
Also when Sync Library is enabled, deleted songs from one device will delete from the other. Delete music, movies, and TV shows from your device
If you do decide to no longer use Apple Music, make sure to cancel your subscription at least 24 hours prior to the renewal date.
See or cancel your subscriptions
Any music downloaded from the Apple Music catalog will be removed from your devices.
Enjoy your day and take care!